Python: ImportError: No module named backports.functools_lru_cache
## Error Description
```sh
File "/home/yubao/data/software/rgbd_benchmark_tools/scripts/evaluate_ate.py", line 210, in
import matplotlib
File "/usr/lib/python2.7/dist-packages/matplotlib/__init__.py", line 127, in
from matplotlib.rcsetup import defaultParams, validate_backend, cycler
File "/usr/lib/python2.7/dist-packages/matplotlib/rcsetup.py", line 29, in
from matplotlib.fontconfig_pattern import parse_fontconfig_pattern
File "/usr/lib/python2.7/dist-packages/matplotlib/fontconfig_pattern.py", line 32, in
from backports.functools_lru_cache import lru_cache
ImportError: No module named backports.functools_lru_cache
```
## Environment
- Ubuntu 18
- Python 2.7
## Solution
```sh
pip install matplotlib==2.0.2
```
## Reference
- https://stackoverflow.com/questions/47179433/python-2-7-functools-lru-cache-does-not-import-although-installed
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